Silk 700

The Silk 700 was one man’s lifelong ambition; to revive the glory that was once Scott motorcycles.

Alfred Angus Scott was a Victorian visionary who almost single-handedly invented the large capacity two stroke motorcycle, then went off to do something odd with sidecars.

Some 60 years later, George Silk took the canted forwards, deflector piston, water cooled, two stroke engine concept and applied it to the 1970s touring market. It almost worked, but rocketing petrol prices and new emissions laws pulled the plug on the high quality Silk 700 before it got going. Great idea, wrong decade.

Long ago, bikes were manufactured in the North of England. Marques like Phelan & Moore made Panthers in Cleckheaton, Dot were in Manchester, Scott´s smoked out of Shipley . This is the story of last roadgoing big capacity two stroke manufactured in England; Silk of Derbyshire.

Like many stories from the pioneer days of motorcycling, this is all about one man´s vision; a dream of taking on the world´s best manufacturers with a truly unique design. It was remarkable that even as the mainstream British bike makers like Norton and Triumph were sliding into bankruptcy during the 1970s, a small band of craftsmen were still able to source materials to start manufacturing.

If you have an interest in how bikes are made, and in particular the weird cult of the two stroke engine, then the Silk story is one that will fascinate you. It is also a typical British tale of the underdog, struggling to succeed against all the odds.

The Silk 700 was one man´s passion, a lifelong dream to re-invent the yowling Scott from his youth. George Silk had restored many 596cc twin cylinder, water cooled Scotts for classic enthusiasts and after a successful performance by a `Silk Special´ at the Barbon Hill Climb in 1970, he started low volume, handbuilt manufacture.

Why Scott ? Well Alfred Angus Scott was a Victorian visionary who designed a sort of prototype Yamaha 350LC back in 1908. With a canted forward 323cc engine, featuring a water cooled upper cylinder block and fuel efficient deflector type pistons, all set in a lightweight cradle frame, the bike dominated road racing so quickly the ACU banned it to give four strokes a chance.

Sadly, the Scott never really developed its full potential, especially after Alf Scott himself went off to make a strange sidecar contraption called a `The Sociable ‘ in the early 1920s. Scott´s finally ceased bike production in the 1950s, when anyone not making dull four stroke singles was on a sticky wicket in the UK market.

The 1970s Silk project was a 653cc updated version of the basic Scott twin cylinder power unit, set in a beautifully crafted steel tubular frame made by Spondon of Derbyshire, who also provided the forks.

George Silk, backed by a Kendal based company called Furmanite, started serious production in 1977, right in the midst of the OPEC oil crises and petrol ration books of the era. It wasn´t a good time to sell a two stroke, especially a handbuilt machine which retailed for £1,984. Sounds peanuts now, but a new 1977 Suzuki GS750 cost £1,285 and was 20mph faster. But George Silk believed his chassis was way superior to the Superbikes of the 70s. Journalists had been harangued for some years regarding the virtues of lightweight two strokes in stiff tubular frames by George, and some went along with the idea.

But the Silk was essentially, an update on a very old engine design, and unlike the Yamaha RD series, or Kawasaki KH triples, the Silk had no race pedigree, and it was marketed as a gentleman´s tourer, which was its great undoing.

In those days, UK bikers aged 25 plus were a bit thin on the ground and young riders hadn’t much hope of raising the £2,000 asking price. However, the Silk 700S was produced at the rate of two machines a week from the Darley Abbey works in Derbyshire in the late 1970s. Customers could choose from five basic paint options; British Racing Green, metallic blue or green, black with gold coachlines or plain red. There was also a special edition in purple and cream, the old Scott colours and co-incidentally, a similar scheme to Silk Cut fag packets. The Silk engine benefited from advances in two stroke technology by Dr Gordon Blair, from Queen´s University Belfast - who were leaders in the field and even had Honda and Suzuki knocking on the lecture theatre doors.

The thermo-syphon cooling system, ( used then by Zundapp in Germany, as well as Silk ) essentially boiled water using engine heat, then fed it back down from the radiator in a rubber tube to the engine cases, where it slowly boiled up again - no water pump was required.

Silk were an engineering company and made the piston port twin cylinder engine in-house at their Derbyshire base. The pressed up, four roller bearing crank had the primary drive taken from the crankshaft centre, to an old Velocette Venom four speed gearbox, which was fitted upside down, so you changed down on the lever to go up the box, like a racebike in fact.

Very odd for most riders. But there was more eccentricity. The engine ran on 50:1 petroil mix, yet had a huge separate three and a quarter pint oil tank reserved for main bearing lubrication, pumped by Silk´s self designed oil pump, which was linked to the throttle. Open the throttle and the oil flowed faster. Seldom was any two-stroke so well lubricated.

Another quirky touch was that the petrol tap was located behind one of the sidepanels, which had to be removed to operate it - not easy switching onto reserve on the M62 then...

The engine´s claimed 48bhp was developed at a mere 6,000rpm, giving the bike touring performance. Peak torque was made at just 3,000rpm, similar to the Suzuki GT750 `Kettle´ of the 70s, and the water jacket gave the unit a strange `woofling´ sort of noise. The twin siamesed exhaust pipes ran into a Spanish made Ossa silencer, which was set so low that spirited cornering could soon ground it - no bad thing on the incredibly skinny ( by modern standards ) tyres fitted.

Wheels were originally 18 inch Borrani alloy rims on early Silks, with six spoke Campagnolo cast items in later 1978 model. The whole bike only weighed 305lbs dry and could hit 115mph, the same as a Z650 Kawasaki which weighed 465lbs.

Silk buyers could specify their bikes to suit their tastes and Neil´s had the cast wheels, with the larger four gallon fuel tank. It also had the extra Lockheed front disc brake, but all Silks had a rear drum fitted. The final drive chain was invariably enclosed, as were the front forks and Girling shock absorbers. Other high standard spec bits included Lumenition electronic ingnition, a quartz halogen headlight, Renolds chains for primary and final drive, plus tapered roller steering head bearings.

Like Alfred Scott, George Silk was reaching for a higher level of quality. With the Silk 700, George and his partners were striving to pay homage to a great Victorian engineer who had put the two stroke on the motorcycling map. They wanted to create a versatile, economical, lightweight machine for older enthusiasts who placed individuality and durability above all else.

But their machine was too faithful to Scott´s eccentric, purist nature and was already ten years too late for the UK market. The era of cheap petrol and oil had gone forever. Big two strokes were an endangered species as US driven emissions laws began to be adopted in Europe too by the early 1980s.Although Suzuki´s GT750 two stroke outsold the CB750 for a couple of years in Britain in the late 1970s, it was perhaps due more to Barry Sheene´s hero status, rather than the bike´s surprisingly civilised touring performance. Finally, Furmanite ordered Silk 700S production to halt in 1981 when UK bike sales collapsed as the Thatcher recession lowered wages and inflation, via mass unemployment.

Specs

Engine Two stroke, twin cylinder, water cooled
cc 653cc
Claimed power (bhp) 48bhp @ 6500
Compression ratio 8:1
Transmission Four speed gearbox, wet clutch, enclosed chain final drive
Cycle parts
Frame; Tubular steel cradle type, by Spondon of Derby
Suspension; 39mm gaitered telescopi front forks, enclosed Girling twin shocks rear
Wheels/Tyres; 3.60 X18 inch front, 4.10 X 18 inch rear, Dunlop TT100s
Brakes; 254mm front disc, twin piston caliper, 180mm drum rear
Dry weight 305lbs
Single Amal Mk 2
Performance
Top speed
fuel capacity 4 gallons

Source carolenash.com

Silk 700 - The Ultimate English Two-Stroke

Source Odd Bikes

Silk 700S Sabre Motorcycle
Image Source Think of the icons of British motorcycling and odds are you will think of one thing exclusively  four-strokes. All the great flagship cycles of English industry – the Commandos, the Manxes, Bonnevilles, Tigers, Interceptors, Gold Stars, and anything else of note from the golden age of British bikes was going to be operating on the principles of suck-squish-bang-blow. British two-strokes were relegated to small, cheap, entry-level machines that were aspired to by no one. Dirty two-strokes were the domain of the Japanese as far as most of the British marques were concerned.

There was, however, one important exception: in the earliest years of British motoring, the Scott Motor Cycle Company produced the fastest and most sophisticated two-stroke machines of the period. Seventy years later, in the waning years of the British cycle industry, George Silk would take that same engine and attempt to build one of the ultimate road-going British two-strokes that would offer a far more refined alternative to those cheap and fast Japanese 'smokers.

The story of the Silk motorcycle must inevitably begin with the Scott. Founded in 1908 by Alfred Angas Scott, the company produced a line of well-developed and advanced two-stroke parallel twins that would become famous for their speed and sophistication during the Edwardian period. Scott was a remarkably talented engineer who began designing two-stroke engines in the late 19th century while working in the shipbuilding industry. His initial designs were for marine engines operating on Joseph Day’s piston-port crankcase-compression two-stroke principles, but he also developed many prescient mechanisms that would later find their way into motorcycle design. You might be familiar with one of his devices: he designed a clever ratchet and pawl mechanism driven by a foot-operated lever for starting a motorcycle engine. That’s right, Scott helped design the kickstart, which would become the most elegant and masculine way to break your own foot outside of a bullfighting ring.

The first production Scott motorcycles featured a 450cc parallel twin, utilizing cross-flow scavenging and liquid cooling, placed in a patented triangulated steel duplex frame of Scott’s own design. The Scott would prove to be one of the most advanced designs of the Edwardian era: aside from kickstarting, a well-braced frame and liquid cooling, the Scott featured telescopic front forks, and his machines were the first to incorporate a multi-speed transmission – only two speeds, but still twice as many as the competition.

Alfred Scott designed from first principles and built his machines to be the best from the outset – his work was free of influence from “tradition”, however recent such conventions may have been in the in nineteen-oughts. But being the most advanced on the market is pointless if your product doesn’t perform, or dun-blows-up as soon as you wind out the throttle. Scott understood this, and designed his namesake machine to be durable, reliable, and well developed. He would prove his machine’s prowess in the traditional, time-honoured manner – by kicking the competition’s ass on the racetrack. In fact Scott motorcycles dominated British road racing and so thoroughly trounced their four-stroke opposition in the 1910s that organizers declared them “overly efficient” and slapped them with a 1.32-multiplier displacement penalty (thus a Scott of 450cc would be forced to compete against 600cc four-strokes). You know you are doing something right in racing when they rewrite the rulebooks to handicap you.

Competition success begat a loyal following and a series of increasingly powerful and advanced models, culminating in the famous Squirrel series of 500 and 600cc machines introduced in the 1920s. Scott also developed a highly advanced manufacturing facility that would be considered state of the art, even after the Second World War – some 30 years after it was initially devised. Unfortunately Alfred Scott passed away in 1923 after contracting pneumonia while testing his Sociable three-wheeled vehicle. He had effectively left the motorcycle business in 1917 to focus on the Sociable project, an unsuccessful attempt to build an inexpensive and practical vehicle for the masses. Without his leadership, the company became technically stagnant and continued producing warmed-over Squirrels for decades with minimal development – what had once been 20 years ahead of the competition was now 20 years behind. Supposed improvements led to more weight and more complexity, which led to outcry from traditionalists. Rather than rework the designs to move forward, Scott continued producing old models alongside the “improved” versions to satisfy their nostalgic followers. In reality even the “improved” designs remained that of the interwar Squirrel series with modest improvements. Evolution was extremely slow and conservative, and as they years passed Scott went from being a highly advanced manufacturer to a curious producer of relics from a bygone era.

Things fizzled out by 1950, at which point the Scott name and facilities were bought out by engineer/investor Matt Holder, who would go on to purchase the rights to Vincent, Velocette and Royal Enfield. Holder moved production from the traditional Scott home of Shipley to Birmingham and restarted production with some improved chassis powered by the same “long-stroke” twin that remained more or less unchanged since 1928. “Birmingham” Scotts were produced in limited quantities until the end of the 1960s, when the works was mothballed and put into storage at the old Triumph Number 2 factory in Meriden. A few bikes would be assembled to order until 1979 and Holder maintained his ownership of the copyrights and spares stock, but series production ended in 1968-69.

Even in the 1960s Scott ‘cycles remained well respected with a loyal following of die-hard owners who extolled the virtues of their advanced design, light weight, and smooth, torquey engines. George Silk was one such die-hard, a man who loved Scotts and who was well known for his exploits in the Scott enthusiast’s community. Silk believed the iconic sloping-twin could be improved with an up-to-date chassis and the application of some modern tuning techniques – after all, the Scott twin hadn’t been updated in 40 years and was initially designed to run on 60 octane fuel. So George set about building a properly modern home for the well-respected parallel twin with its smooth, torquey powerband.

Yes, torquey – unlike more recent two-stroke designs, Scotts utilized a cross-flow scavenging setup that produced a wide powerband at the expense of top-end performance. In two-strokes, with both the exhaust and intake ports open simultaneously on the power stroke, the flow of mixture across the cylinder (called scavenging) must be controlled in some fashion. Too much flow without enough control will result in over scavenging, where the unburned mixture will just flow straight through into the exhaust. Cross-flow (also referred to as piston-port) scavenging uses a tidal-wave shaped deflector cast into the crown of the piston to direct the flow of incoming mixture upward into the roof of the combustion chamber, swirling it up and over the crown and past the spark plug.
Two-Stroke Cross-Flow Scavenging

Cross-flow designs have been considered obsolete for a quite some time. Modern high performance two strokes utilize Schnuerle porting, better known as loop scavenging, developed in 1926 by German diesel engine engineer Adolf Schnürle and first adopted in motorcycles by DkW in 1934. Loop scavenging uses carefully arranged and angled ports to accomplish the swirling effect needed for proper performance, without the need for a deflector. Thus the piston can be flat topped, reducing reciprocating mass and allowing for a higher compression ratio – the tall deflector on a cross flow piston acts as a heat sink which increases the likelihood of detonation and piston overheating, thereby limiting potential tuning. There are, however, two distinct advantages to cross-flow engines – one is a much wider powerband and a flatter torque curve than a loop scavenged engine, the other is excellent fuel economy.
 

That’s enough two-stroke theory, back to story at hand: George Silk Jr. was the son of an ardent Scott fan who had George apprentice under Tom Ward, a technician who had worked for Scott prior to the First World War. Tom wasn’t able to pay George an adequate salary once his apprenticeship was over, so George entered into the commercial engine rebuilding business at age 21. He tinkered with Scott tuning on the side, with some attempts at building competitive vintage racers that he campaigned in sprint and road racing in the late 1960s. While working on these racers, he formulated an idea to place the classic Scott engine into a modern frame to improve handling, and idea that would become the genesis of the Silk motorcycle.

George approached Bob Stevenson at famed frame manufacturer Spondon to modify a duplex cradle frame to fit a Scott engine attached to a Velocette four-speed gearbox. The resulting machine had tidy handling, but was let down by the ancient motor design. Even the best Scotts were lucky to crack a genuine 20 hp without serious reworking, so placing such a motor into a modern road-racing frame was certainly overkill, but the results were encouraging.

Spurred on by the handling offered by his Spondon-framed prototype, George set about building a series of Silk-Scott Specials for discerning clients. He founded Silk Engineering with partner Maurice Patey in Derbyshire in 1971 and began to mate hotted-up Scott motors to Spondon frames with modern running gear. However, George hadn’t obtained the rights to the Scott name from copyright owner Matt Holder. Legend has it that some overly proud owners may have exhibited a bit too much arrogance regarding their new and improved Scotts, which led to Holder disavowing the endeavour and refusing to sell Scott spares to Silk. Another telling of the story says that Holder was annoyed that Silk was putting Scott logos onto the tanks of his wholly unauthorized machines, and swore him off. Whatever the circumstances, this meant that prospective Silk-Scott buyers were asked to supply their own crankcases to be able to construct the machines, no mean feat considering that Scotts were prized machines that were unlikely to be gutted to build a hybrid.

Silk applied improvements to the engine as well as to the chassis - His experience under Tom Ward and his years of building racers had given him some ideas on how to improve the performance of Scott engines. Porting was modified, the crankshafts and bottom ends were improved, and cutaways were added to the solid-skirt pistons to improve flow from the crankcase into the transfer ports. The basic bore, stroke and timing of the donor engines were retained, but aside from that the motors were more or less new from the crankcases up. The delicate Scott radiator was replaced with a more robust and easily found unit taken from the Velocette LE. A four-speed Velocette gearbox was adapted to work with the Scott motor – it was flipped over and mated to the Scott engine via a chain primary and clutch designed by Silk. Being flipped upside down, the shifting was left hand, reverse pattern. The box had to be lubricated with the outmoded method of manually greasing the gears. At least one customer returned the machine to have the gearbox sealed and run in oil to make maintenance a bit less… intensive.

These Silk-Scott hybrids were produced according to customer specifications from 1971 to 1975 - total production during was a mere 21 machines. However original Scott engines and crankcases were extremely hard to obtain in good workable order, and Matt Holder’s blockade precluded the purchasing of new stock parts, so a solution was devised. In 1975 George announced a new, street-legal Silk machine would be built with a proprietary engine, designed and produced in-house to circumvent the Holder blockade. The Silk 700S would be heavily inspired by the design of the Scott parallel twin, but would feature many modern improvements and much higher performance to match the excellent chassis they had developed. Initial price tag was £1355, or around 150% the price of a contemporary Norton Commando, making it the most expensive production bike on the market at the time.
Silk 700S Mark 2 Engine

David Midgelow, a Rolls Royce engineer and a neighbour of George’s, was contracted to design the engine. Dr. Gordon Blair was hired to design the porting, exhaust, and optimize flow characteristics. The basic Scott design tenets were retained in the new engine – it was a 180-degree parallel twin sloped forward 40 degrees, with cross-flow scavenging, and liquid cooling via a pumpless thermo-syphon system that used convection to circulate coolant. The cases, heads and cylinders were cast in aluminum alloy, with cast iron bore liners. A 76mm bore and 72mm stroke gave 653cc. The deflector-crowned pistons were externally copied from a 1928 Scott sample, with a different internal shape and skirt cutouts. Because piston manufacturer Hepolite required a minimum order for custom pistons and Silk was such a tiny manufacturer, only 1000 of the 700S pistons were cast and all were in the same 76mm diameter. No oversized pistons were available for rebuilds – if you need a rebore, you need to get the iron liners replaced to maintain the stock piston diameter.

Fueling was via a single 32mm Amal Concentric running in the traditional Scott location, feeding an inlet tract cast into the crankcase between the two cylinders. A proprietary oil injection system was used that where oil was metered out as needed from an alloy tank under the seat. A device connected to the throttle dispensed oil according to how much throttle was applied, allowing for up to 1000 miles between oil tank fill-ups, with no need to premix gas and oil. Like the Silk-Scott Special the piston skirts had cutaways to allow better flow through the crankcase into the transfer ports, with the bonus of fresh charge cooling the underside of the piston crown and lubricating the small end of the con rod. At first glance most two-stroke savvy people assume the cutouts indicate a reed valve inlet but in fact they only affect the transfer ports, not the crankcase intake.

Claimed power for the new mill was 45 bhp at a leisurely 6000 rpm, with 45 lb-ft available at only 3000 rpm. While not earth shattering by modern standard, you must keep in mind that an 850 Commando of the period made a claimed 53hp and weighed over 100lbs more than the Silk. Additionally, fuel economy was in the 50 plus mile per gallon range, exceptional for a two-stroke of any description. A loop-scavenged machine might have the edge in power - the 653cc Silk made about the same horsepower as a contemporary Yamaha RD350 – but a Silk would outgun the Asian ‘smoker in the grunt department. The Silk also had a characteristically throaty growl that is unusual for a two-stroke, helped in part by its “siamesed” two-into-one exhaust – it almost sounds like a four-stroke machine, but not quite.

Like the Silk-Scott Special a Velocette-type four-speed gearbox with gearsets cut by Roger Moss was used, but in the 700S it was integrated into the newly designed crankcases. Final drive was by chain and sprocket, but fully enclosed against the elements. Silk, ever the salesman, claimed that the spread of power was sufficient to not need more than four gears, unlike those peaky Japanese machines that required five or six gears. Indeed, reading the sales brochure for the Silk you would think it was the finest, most carefully designed and magnificently constructed machine of all time, such was the care and attention that was put into the hyperbolic descriptions. You have to respect the company’s enthusiasm, and the fact that they were able to design and build their own motor despite being a tiny, underfunded operation.

Chassis-wise the Silk 700S had a duplex Spondon frame similar to the Silk-Scott Special. Spondon also provided the front forks, which were patterned after Ceriani items. The rear swingarm was a traditional unbraced tube-section arm suspended by a pair of Girling shocks. Borrani wire wheels were standard. Brakes were Lockheed or Spondon hydraulic disc with a cast iron rotor at the front, and a drum at the rear. A double disc setup at the front was optional - you could also substitute a twin-leading shoe drum if you were leery of that newfangled disc brake witchcraft. The bodywork was similar to that of the Silk-Scott Special, with a slab-sided aluminum fuel tank (available in 14 and 18 litre displacement) and locking sidepanels. The resulting machine was cleanly styled and quite modern looking. It was also remarkably light weight – only 305 lbs dry, featherweight for a liquid-cooled 650cc machine. This, along with the well-developed chassis, contributed to excellent handling that was praised by reviewers and owners alike.

Production was slow but steady, and in 1976 the operation was taken over by Furmanite International Group. 1977 saw the introduction of an updated Mark 2 700S dubbed the “Sabre”. The engine compression was raised slightly to bump power to a claimed 48 bhp, and the engine cases were given a cosmetic redesign with finned cylinder jackets. A new seat and instruments were also installed. Campagnolo cast wheels were an available option, and with these installed the Silk looked like a thoroughly modern machine that belied its antiquated roots. If they were expensive at the beginning of production in 1975, they were breathtakingly priced by the time production ended in 1979 – at which point the retail was nearly £2500. The operation ceased in the face of steady losses (at least £200 per machine, despite nearly doubling in price since 1975) and increasingly strict emissions laws on the horizon threatening the very existence of street-legal two-strokes.

Silk production ended after only 138 examples were produced, making the 700S a rare and highly coveted machine that is highly prized among British bike aficionados. It is exceptional when a Silk-Scott Special or a 700S changes hands, and owners are a dedicated lot who hold their bikes in high esteem. After all, what other British machine offers some of the best handling available in the 1970s in an exceptionally lightweight package with sophisticated two-stroke power? The Silk was the product of George Silk’s enthusiasm for the refinement and potential of the venerable Scott engine, dropped into a modern chassis that could challenge the traditional four-stroke hierarchy in Britain.

Source Odd Bikes